1. Field of the Invention
The present invention relates to a nitrogen oxides (NOx) decreasing apparatus for an internal combustion engine. More specifically, the present invention relates to an exhaust gas purification system with a lean NOx catalyst capable of reducing NOx included in exhaust gas from an internal combustion engine, such as a so-called lean burn gasoline engine, a diesel engine, or a hydrogen engine, independently of a concentration of oxygen (O.sub.2) included in exhaust gas, that is, independently of an operating air-fuel ratio.
2. Description of the Prior Art
As a method for decreasing NOx included in exhaust gas from an internal combustion engine, the following three methods are known:
(1) to decrease NOx by using a three-way catalyst, PA1 (2) to decrease NOx by using combustion at excessively lean air-fuel ratios, and PA1 (3) to decrease NOx by using a lean NOx catalyst (as disclosed in Japanese Patent Publication HEI 1-139145). However, there are the following problems with these methods.
With method (1), the ratio of air to fuel supplied to an engine must be controlled to about 14.5, namely, the stoichiometric air-fuel ratio. If the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, NOx cannot be decreased by the three-way catalyst. Contrarily, it is well known that to obtain good fuel economy, an engine should be operated at air-fuel ratios leaner than the stoichiometric air-fuel ratio, as shown in FIG. 2. Thus, NOx decrease and fuel economy are incompatible in method (1).
Method (2) seeks to combine NOx decrease with fuel economy by using a so-called lean burn engine. However, when the engine is operated at air-fuel ratios lean enough to decrease NOx, the combustion approaches a misfire limit, and driveability gets worse. To prevent this, an improvement has been proposed, wherein turbulences are generated within an engine cylinder so that the burning velocity is increased to thereby shift the misfire limit to the lean side. However, if the turbulences are excessive and the flow velocity becomes too high, formation of a flame core and propagation of the flame in an early period of combustion will be obstructed. Thus, there is a limit to this method. Also, another improvement has been proposed, wherein the air-fuel ratio distribution within an engine cylinder is controlled so that rich air-fuel mixtures are formed only in a region close to the ignition plug to produce easy ignition. However, as illustrated in FIG. 3, when the misfire limit is shifted to the lean side, the negative slope of the NOx concentration curve also is decreased. Thus, not much effect on NOx concentration can be expected.
Method (3) is intended to solve the above-described problems of item (2), more particularly, to operate an engine at air-fuel ratios slightly closer to the stoichiometric air-fuel ratio than the misfire limit and then to purify the insufficiently decreased NOx by using a zeolite-type lean NOx catalyst. This method has the potential to provide a clean system that also has good fuel economy. However, since the lean NOx catalyst can operate only under oxidizing exhaust gas conditions and is usually exposed to high temperatures, it is difficult to obtain both a sufficiently high NOx conversion by the lean NOx catalyst and durability of the catalyst.
As will be understood from the above, problems of practical use exist with any conventional NOx decreasing method.
Lean burn gasoline engines as well as diesel engines include excess O.sub.2 in the exhaust gas, that is, they are operated under oxidizing gas conditions. The leaner the air-fuel ratio, the greater is the concentration of O.sub.2 included in the exhaust gas. A catalyst which reduces NOx under oxidizing gas conditions is defined as a lean NOx catalyst, which is usually composed of a noble metal-type catalyst or a zeolite-type catalyst. The lean NOx catalyst has an NOx conversion versus temperature characteristic as shown in FIG. 4. At temperatures above 350.degree. C., mainly reduction of NOx by HC occurs, while at low temperatures below 250.degree.-350.degree. C., reduction of NOx by hydrogen (H.sub.2) occurs, wherein NOx purification by H.sub.2 is possible.
However, since the lean NOx catalyst is usually installed in or near an engine exhaust manifold in a conventional exhaust system, the temperature to which the catalyst is exposed is as high as 800.degree.-900.degree. C. Further, since the lean burn engine is operated at above stoichiometric air-fuel ratios, almost no H.sub.2 remains in the exhaust gas. Therefore, the NOx reduction characteristic of a lean NOx catalyst at low temperatures below 250.degree.-350.degree. C. has not been used in a conventional lean burn gasoline engine or diesel engine.